Feedback 101

My kids are homeschooled and meet regularly with other homeschoolers.  Recently they decided to put on a talent show and I had the opportunity to sit behind the sound board for the event.  Unfortunately I couldn’t make it to the dress rehearsal because of work conflicts, so I had to wing it when I did finally get there.  My wife was involved and warned me that one of the kids who was wearing a wireless microphone might approach another microphone during a drama, her fear being that the two microphones meeting would cause feedback. 

 That’s the inspiriation for starting off with a post on where feedback comes from, because two microphones meeting does not cause it (but two microphones can be out of phase and cause an entirely different problem…a later discussion).  Being too close or too far from the microphone doesn’t cause feedback (but it is a factor). 

Feedback is a result of an output and input crossing paths when they shouldn’t.  A microphone is an input, and a speaker is an output.  If you put the two together, you can end up with feedback.  What happens is that a sound comes out of the speaker, gets picked up by the microphone, goes out of the speaker, gets back in the microphone, and this loop happens so fast that you end up with a loud noise that gets louder and louder. 

And it isn’t just microphones that can cause this.  Sometimes guitar pickups can cause feedback.  Another source can be computers.  I use my laptop for just about everything when I’m working on the production side of an event, I record the event, play pre-show music, sometimes play video clips, etc.  Recording and playback at the same time works fine, unless I forget to mute the Line In…that’s when the audio coming in (the record feed) goes right back out the computers output, which in turn goes back into the record feed, and you get a somewhat loud constant beep.  It doesn’t sound like regular feedback because the volume is fixed at whatever the computer and record feed are set to, there are no variables of distance between speaker and microphone. 

I could write pages and pages on what to do to prevent feedback, how equipment such as compressors can intervene, and so forth.  Needless to say I will revisit this topic again.  But here’s some tips to start off with:

When a person is holding a microphone that starts to feedback, their natural response is to pull the microphone away from them, or they might think they are speaking too loudly.  But the opposite may be true, because the speaker/singer is too quiet, the sound guy turns up the volume, which makes the microphone more sensitive and possibly start a feedback loop.  If the speaker/singer can be louder, the volume on the console can be turned down, and the loop be broken.

If there is a speaker facing the microphone (monitors, or house speakers if the person is in the audience), it may be necessary to turn down those speakers to control the feedback.  For example, if a person speaking is out in the audience and walks to the left side of the room in front of the main speakers, the sound guy can turn the PAN dial (kind of like a balance, tells the channel which speaker to come out of) to the right speakers so that the left speakers are quieter, and the right speakers compensate. 

Once you become more familiar with compressors and equalizers, those will also be your best tools in preventing/controlling feedback.  For example, in certain environments you can have problem frequencies, certain tones or pitches reflect off the wall or ceiling back to a microphone sooner than others.  An EQ will allow you to turn down just those frequencies without having to turn down the whole volume.

The kids talent show went well by the way.  We didn’t have the proper microphones to amplify the drama, but the audience was small and could hear the kids fine, and fortunately I had a separate volume on the record feed so that I could turn up the mic volume on the recording without risking feedback from the main speakers. 

Your questions and suggestions are welcome, so if I wasn’t clear on anything or if you have a preference on the next topic, please feel free to comment or contact me directly (my contact info is on the about page).